The C1q precipitin test was performed in serum samples from five groups of patients: (1) 20 patients with acomplementemic systemic lupus erythematosus glomerulonephritis (SLE), (2) 2 patients with serum sickness due to the administration of horse serum, (3) 2 patients with serum sickness preceding hepatitis B, (4) 50 patients with chronic urticaria, and (5) 30 normal controls. Positive C1q precipitin tests were found in all patients with SLE and the four cases of serum sickness. Positive tests correlated with depressed serum complement (C3 and C4) levels and were found only in the early phase of serum sickness. Urticaria patients uniformly had negative C1q precipitin tests and normal serum complement levels.